1. Field of the Invention
The present invention relates to a metallic separator which is installed in a proton-exchange membrane fuel cell and a method for manufacturing the same metallic separator.
2. Description of the Related Art
In a proton-exchange membrane fuel cell, a laminated body in which separators are laminated on both sides of a flat plate-like membrane electrode assembly (MEA) is made to be one unit, and a plurality of units are stacked together to form a fuel cell stack. The membrane electrode assembly is of a three-layer construction in which an electrolytic membrane comprising an ion-exchanging resin as held between a pair of gas diffusion electrodes which constitute a positive pole (cathode) and a negative pole (anode). The gas diffusion electrode is such that a gas diffusion layer is formed on the outside of an electrode catalyst layer which contacts the electrolytic membrane. In addition, the separator is laminated in such a manner as to be brought into contact with the gas diffusion electrode of the membrane electrode assembly, whereby a gas flow path through which gas is allowed to flow and a coolant flow path are formed between the separator so laminated and the gas diffusion electrode. According to the fuel cell, for example, when hydrogen gas which is fuel is allowed to flow through a gas flow path which faces the gas diffusion electrode on the negative pole side, whereas oxide gas such as oxygen or air is allowed to flow trough a gas flow path which faces the gas diffusion electrode on the positive pole side, an electrochemical reaction occurs and current is generated.
The separators need to have a function to supply electrons generated through a catalytic reaction of hydrogen gas on the negative pole side to an outside circuit, as well as a function to supply electrons from the outside circuit to the positive pole side. To this end, conductive materials, such as graphite and metallic materials, are used for the separators. In particular, the metallic materials are considered to be advantageous in that they have superior mechanical strength and that they can be formed into a thin plate which can eventually provide a separator light in weight and small in size. As the metallic separator, there is a separator which is manufactured by rolling stainless steel containing conductive inclusions which are deposited and/or dispersed therein into a thin plate, and forming by pressing the thin plate so as to have a cross section constituted by alternate ridges and grooves so that the grooves formed on front and back surfaces of the thin plate are used for the gas flow paths and coolant flow paths. The conductive inclusions are such as to contribute to the improvement in power generating performance by forming a conductive path.
With the metallic separators so constructed, the ridges surfaces are brought into contact with gas diffusion electrodes of the membrane electrode assembly in a state in which the separators are assembled to the membrane electrode assembly. The ridged portions are formed into a trapezoid having sides which are slightly inclined so that the separator can easily be removed from the die after pressing. In addition, corners which are transitional portions from the surface of the ridged portion to the sides are inevitably formed into a round shape (R-shape) by bending. These constitute restrictions on the enlargement of actual contact areas to the membrane electrode assembly at the surfaces of the ridged portions. A reduction in contact area of the separator to the membrane electrode assembly leads to an increase in contact resistance and prevents the improvement of power generating performance. Therefore, the enlargement of the contact area is desired. In addition, some of separator, the surfaces of the ridged portions are each close to the round shape as a whole and hence their flattened surfaces become limited. As this occurs, it is difficult to ensure that a desired surface pressure is obtained at the surfaces the ridged portions which are in contact with the membrane electrode assembly, this also leading to an increase in contact resistance.
Further, when stainless steel in which conductive inclusions are deposited and/or dispersed is rolled into a thin plate, there may be caused a case where an abnormal layer is produced on the surface of the thin plate in which conductive inclusions which are broken extremely finely by rolling are caused to aggregate. In case a fuel cell is constituted by separators in which the abnormal layers exist on the surfaces thereof and is then put in operation, the conductive inclusions existing in the abnormal layers drop, which leads to deterioration in power generating performance.
Moreover, in the manufacture of separators as has been described above, since there exists a surface rolling-affected layer on a stainless steel plate, the steps are required of grinding to remove the surface rolling-affected layer so as to allow good conductive inclusions that have not been affected by rolling to be exposed on the surface of a base metal and, furthermore, allowing the exposed conductive inclusions to protrude so as to reduce the contact resistance. However, there has existed a problem that a naturally oxidized film is formed on the surface of the base metal between the step of grinding and removing the surface rolling-affected layer and the step of allowing the conductive inclusions to protrude. Once a naturally oxidized film is formed on the surface of the base metal, even if the step of allowing the conductive inclusions to protrude is implemented thereafter, the effect on the improvement in conductivity by the step of allowing the conductive inclusion to protrude cannot be obtained sufficiently due to the existence of the naturally oxidized film. Owing to this, in order to obtain sufficient conductivity, a complicated step must be implemented further, leading to another problem that the production costs are increased.
Further, after the process of grinding to remove the surface rolling-affected layers so that the conductive inclusions are allowed to protrude in the vicinity of the surfaces of the stainless steel plate to thereby reduce the contact resistance, a process is conducted of applying to newly produced surfaces of the stainless steel plate a treatment for providing corrosion resistance thereto. In related art, the passivation treatment has been used for providing the corrosion resistance to the newly produced surfaces. However, there has been existing a problem that a naturally oxidized film is formed on the newly produced surface during the passivation treatment. The naturally oxidized film is inferior to a film in a passive state in corrosion resistance, and therefore, a further provision of corrosion resistance has been required. However, even if the passivation treatment is implemented after a naturally oxidized film has been formed, the naturally oxidized film interrupts the passivation of the newly produced surface, and therefore, the corrosion resistance improvement effect by the passivation treatment cannot be attained sufficiently. Due to this, in order to obtain a sufficient corrosion resistance, a further complicated step has to be implemented, this leading to another problem that the production costs are increased.